Tuning and Enhancing White Light Emission of II–VI Based Inorganic–Organic Hybrid Semiconductors as Single-Phased Phosphors

Single-phased white light emitters made of semiconductor bulk materials are most desirable for use in white light-emitting diodes (WLEDs) based on both photoluminescence and electroluminescence. Here we demonstrate Cd and/or Se substituted double-layer [Zn₂S₂(ha)] (ha = <i>n</i>-hexylamine) hybrid semiconductors emit bright white light in the bulk form and their emission properties are systematically tunable. The ternary Zn_{2–2<i>x</i>}Cd_2<i>x</i>S₂(ha) hybrid compounds exhibit two photoluminescence (PL) emission peaks, one of which being attributed to band gap emission, and the other resulting from Cd doping and surface sites. The Cd concentration modulates the optical absorption edge (band gap) and the positions of the two emission bands along with their relative intensities. The ZnS-based hybrid structures (with a nominal Cd mole fraction <i>x</i> = 0.25) emit bright white light with significantly enhanced photoluminescence quantum yield (PLQY) compared to its CdS-based hybrid analogues. For the quaternary Zn_{2–2<i>x</i>}Cd_2<i>x</i>S_{2–2<i>y</i>}Se_2<i>y</i>(ha) compounds (<i>x</i> = 0.25 and different nominal Se mole fractions <i>y</i>) the synergetic effect between doped Cd and Se atoms leads to further tunability in the band gap and emission spectra, yielding well balanced white light of high quantum yield. Detailed analysis reveals that the PL emission properties of the ternary and quaternary hybrid semiconductors originate from their unique double-layered nanostructures that combine the strong quantum confinement effect and large number of surface sites. The white-light emitting hybrid semiconductors represent a new type of single-phased phosphors with great promise for use in WLEDs

Symmetric Confined Growth of Superstructured Vanadium Dioxide Nanonet with a Regular Geometrical Pattern by a Solution Approach

Controllable self-assembly of ordered and regularly patterned semiconductor nanoarchitectures is of great interest in achieving fantastic functionalities and properties of nanomaterials in nanodevices. Here we demonstrate a symmetric confined growth methodology for fabricating a geometrically patterned and well-oriented two-dimensional nanonet by a solution growth. A uniform orthogonal VO₂ nanonet composed of single-crystalline nanowalls is self-assembled in a one-step process and exhibits single-crystal-like crystallographic characteristics. It is revealed that the 4-fold symmetric structure of (001) TiO₂ determines the orthogonal geometrical pattern of the nanonet; in addition, the interfacial mismatch energy controls the horizontal growth direction and morphology of one-dimensional nanocrystals competing with the surface energy. The unique VO₂ nanonet exhibits excellent thermochromic performances due to its self-generated porosity and sluggish phase transition. The initial optical modulation temperature is near room temperature. The solar modulating ability (Δ<i>T</i>_sol) is up to 11.82% with the maximum visible light transmittance (<i>T</i>_vis‑max) more than 70%. The proposed growth strategy could be adopted in more systems to perform self-assembly of regularly patterned nanoarchitectures with well interconnectivity and preferred orientation, which offers promising opportunities for exploiting potential nanodevices in various applications

Single-Phase Lithiation and Delithiation of Simferite Compounds Li(Mg,Mn,Fe)PO₄

Understanding the phase transformation behavior of electrode materials for lithium ion batteries is critical in determining the electrode kinetics and battery performance. Here, we demonstrate the lithiation/delithiation mechanism and electrochemical behavior of the simferite compound, LiMg_0.5Fe_0.3Mn_0.2PO₄. In contrast to the equilibrium two-phase nature of LiFePO₄, LiMg_0.5Fe_0.3Mn_0.2PO₄ undergoes a one-phase reaction mechanism as confirmed by ex situ X-ray diffraction at different states of delithiation and electrochemical measurements. The equilibrium voltage measurement by galvanostatic intermittent titration technique shows a continuous change in voltage at Mn³⁺/Mn²⁺ redox couple with addition of Mg²⁺ in LiMn_0.4Fe_0.6PO₄ olivine structure. There is, however, no significant change in the Fe³⁺/Fe²⁺ redox potential

Electrode Reaction Mechanism of Ag₂VO₂PO₄ Cathode

The high capacity of primary lithium-ion cathode Ag₂VO₂PO₄ is facilitated by both displacement and insertion reaction mechanisms. Whether the Ag extrusion (specifically, Ag reduction with Ag metal displaced from the host crystal) and V reduction are sequential or concurrent remains unclear. A microscopic description of the reaction mechanism is required for developing design rules for new multimechanism cathodes, combining both displacement and insertion reactions. However, the amorphization of Ag₂VO₂PO₄ during lithiation makes the investigation of the electrode reaction mechanism difficult with conventional characterization tools. For addressing this issue, a combination of local probes of pair-distribution function and X-ray spectroscopy were used to obtain a description of the discharge reaction. We determine that the initial reaction is dominated by silver extrusion with vanadium playing a supporting role. Once sufficient Ag has been displaced, the residual Ag⁺ in the host can no longer stabilize the host structure and V–O environment (i.e., onset of amorphization). After amorphization, silver extrusion continues but the vanadium reduction dominates the reaction. As a result, the crossover from primarily silver reduction displacement to vanadium reduction is facilitated by the amorphization that makes vanadium reduction increasingly more favorable

What Happens to LiMnPO₄ upon Chemical Delithiation?

Olivine MnPO₄ is the delithiated phase of the lithium-ion-battery cathode (positive electrode) material LiMnPO₄, which is formed at the end of charge. This phase is metastable under ambient conditions and can only be produced by delithiation of LiMnPO₄. We have revealed the manganese dissolution phenomenon during chemical delithiation of LiMnPO₄, which causes amorphization of olivine MnPO₄. The properties of crystalline MnPO₄ obtained from carbon-coated LiMnPO₄ and of the amorphous product resulting from delithiation of pure LiMnPO₄ were studied and compared. The phosphorus-rich amorphous phases in the latter are considered to be MnHP₂O₇ and MnH₂P₂O₇ from NMR, X-ray absorption spectroscopy, and X-ray photoelectron spectroscopy analysis. The thermal stability of MnPO₄ is significantly higher under high vacuum than at ambient condition, which is shown to be related to surface water removal